Copper smelters today represent a major source of S0.sub.x air pollution emissions. A typical copper smelter may exhaust 200 to 300 tons per day of SO.sub.2 and SO.sub.3 to the atmosphere, and pose very serious local air pollution problems. These off-gases result from conventional pyrometallurgical copper smelter operations including roasters, reverberatory furnaces, and converters.
Currently, the smelter industry is investigating hydrometallurgical processes for production of copper by means of aqueous ammonia solutions. However, if that technology proves feasible it is only a partial answer to the air pollution problem for future plants, and is not intended to cover the problems of retrofit in existing plants employing pyrometallurgical process techniques. It also appears applicable only to chalcocite (and possibly covellite, azurite, chrysocolla and malachite), but not particularly effective for iron-containing ores such as chalcopyrite or bornite. This leaching also is not particularly effective for recovery of gold and silver from ores containing such precious metals.
As for existing pyrometallurgical plants, and those currently planned or under construction, the copper industry is proposing the use of water scrubbers for production of sulfuric acid. However, these acid plants generally require relatively high SO.sub.2 concentration in the flue gases for reasonable efficiency. In addition, they exhaust a tail gas containing sulfurous and sulfuric acid mist which itself poses some pollution problems. The use of acid plants is not generally designed to scavenge SO.sub.2 or SO.sub.3 from low SO.sub.2 concentration gases such as gases coming from reverberatory furnaces and/or roasters. These gases generally contain only 0.5 to 2.5 percent SO.sub.2 which only marginally can be controlled by acid plant type scrubbers. Normally, the scrubbers require the more concentrated off-gases, such as 6 to 12 percent SO.sub.2 -containing converter off-gases.
In addition, the production of large quantities of sulfuric acid can pose a regional market glut of sulfuric acid which cannot be sold or otherwise used. Excess acid must therefore be disposed of. One common method of disposal is to dump the acid in natural limestone quarry formations. However, this itself poses a water pollution problem since limestone normally contains from 1 to 50 percent of magnesium carbonate. The reaction with sulfuric acid produces highly water soluble magnesium sulfate (an epsom salt having known laxative properties).
A certain portion of the acid can be used in heap, dump, inplace, or vat leaching operations. In copper mining operations, the better ore is forwarded to flotation concentrators, and the resulting beneficiated ore is then forwarded to the smelting operations for pyrometallurgical smelting to produce copper. Marginal, and/or submarginal, ore is normally placed on dumps or in heaps on asphalt, concrete, or similar impermeable pads. Presently used dumps range from 5.5 million to 4 billion tons of ore. Sulfuric acid or a liquor of sulfuric acid and ferric iron is introduced into the surface or interior of the dumps or heaps, and the liquor dissolves metals from the minerals. The principal copper oxide minerals dissolved by leaching are azurite, chrysocolla, and malachite. The primary copper sulfide minerals, chalcocite, chalcopyrite, and covellite also are dissolved during leaching. Pyrite is also oxidized during leaching to form ferrous sulphate and sulfuric acid. The effluent from the dumps or heaps forms a pregnant liquor rich in copper as CuSO.sub.4. This pregnant liquor is then reacted with iron, typically in Launders or cone-type reaction tanks, to produce "cement" copper. The iron is generally in the form of tin cans or other types of shredded iron metal. Under the acidic conditions of the reaction, the cuprous ions are reduced to elemental copper which precipitates and is filtered out. The elemental iron is oxidized to ferrous iron and remains behind in the "barren solution." The theoretical iron consumption is 0.88 lb. Fe/1.0 lb. Cu, but in practice the consumption (known as the can factor) ranges from about 1.2-2.5 lbs. Fe/lb. Cu. The fine particulate elemental copper tends to naturally agglomerate into a hard mass, hence its name "cement" copper.
Typical values for copper leaching operations are illustrated by those for the Bingham Canyon, Utah operation:
______________________________________ Barren Liquor Input Pregnant Liquor Parameter Into Dump Effluent From Dump ______________________________________ Maximum Flow Rate, gpm 53,000 53,000 pH 2.8 to 3.0 2.5 H.sub.2 SO.sub.4 Added, g/l 0.1 0 lbs/1000 gal. 0.834 0 Copper Content, g/l 0.12 to 0.18 1.80 lbs/1000 gal. 1.0 to 1.5 15.0 Temperature, .degree. F. 92 to 94 110 to 125 ______________________________________
The presence of chemosynthetic autotrophic bacteria in the leach solutions or within the dumps promotes and accelerates oxidation of iron pyrite to ferric sulphate and sulfuric acid and the oxidation of copper sulfides to copper sulfate. Oxidation rates are known to be accelerated by as much as 2000 to 1 at optimum temperatures of 95.degree. F. (35.degree. C.) for both pyrites and copper sulfides.
The "barren" solution, which is, in fact, rich in ferrous iron values, is then adjusted to a pH of 1.5 to 3.0 with sulfuric acid, and is recycled to the leach dumps. However, excess ferrous iron tends to form ferrous and ferric hydroxides and ferrous sulfate inside pipelines, and on the surface and interiors of the dumps if the pH gets too high. When the solution pH is above 3.0 the ferrous salts precipitate; generally a pH of 2.4 is required to prevent precipitation of salts, while a pH of 2.1 is required to redissolve precipitated salts. Thus, strict control of iron content and pH is necessary to minimize the precipitation of iron salts. But iron salt precipitation within the dumps is extremely difficult to overcome because the formation of impervious layers prevent the movement of leach solutions within the dumps. Copper bearing rock below the layers have no contact with leach solutions and leach efficiency is reduced. In an attempt to improve efficiency, the barren, ferrous-rich iron solutions are often passed to ferrous ponds where the pH is adjusted into the 4 to 5 range so that ferrous/ferric hydroxides and ferrous sulfate can precipitate. From these ponds, the supernatant liquor is readjusted to a pH in the 2 to 3 range and recycled to the dumps or heap leaching operation.
Although sodium alkalis are recognized as being better SO.sub.2 sorbents than calcium alkalis from the point of view of reactivity and not posing scrubber scaling or plugging problems, the high water solubility of the end-product sodium sulfite and sodium sulfate has substantially prevented adoption of sodium alkalis for scrubbing of SO.sub.2 in flue gases. Therefore, there has been an attempt to go to a regeneration type scrubbing operation such as the Wellman-Lord sulfite/bisulfite process, or the double alkali process. The Wellman-Lord process involves scrubbing SO.sub.2 -containing flue gases with bisulfite which combines with the SO.sub.2 to form sodium sulfite. This is then regenerated to the bisulfite form and the scrubbing is repeated. The double alkali process involves the use of sodium within the scrubber. The scrubber liquor is then reacted with calcium oxide (lime) alone or also with calcium carbonate (limestone) in one or more tanks exterior to the scrubber. The sodium values are recycled while calcium sulfite/sulfate sludge is removed.
However, these processes are expensive from the point of view that the capital cost of equipment for regeneration brings the total SO.sub.x control equipment cost to more than double that for scrubbing equipment alone. Further, the sulfite/bisulfite process forms some sodium sulfate crystals which must be continuously bled from the operation. The purge streams amount to 5 to 25 percent of the input sodium, and this material is highly water soluble and must be disposed-of.
As for the double alkali process, the preferred scrubber effluent is sodium sulfite since the sulfite-lime reaction is faster than sulfate-lime. However, sodium sulfate appears predominant under typical scrubber conditions. In addition, the end-product calcium sulfate/sulfite forms a thixotropic sludge which is very difficult and expensive to dispose of since it will not dewater to more than about 50 percent solids. The disposal of the sludge involves transportation of excess water which is expensive.
The sludge cannot be piled above ground since it will not support its own weight. Current disposal involves placing the sludge in clay pits or ponds. However, there is concern over that disposal technique due to the ability of gypsum to increase the porosity of clay. Where the porosity of the clay disposal pond is increased, magnesium sulfate can leach therefrom. Magnesium sulfate is present in sludge because of the 1- 50 % MgCO.sub.3 or MgO present in limestone or lime. In addition, wet scrubbing tends to collect heavy metals which then pose a possiblity of water pollution if they leach from such ponds.
In the hydrometallurgy art, proposals have been made to remove metals, e.g. iron and/or aluminum, alone or concurrently, from copper dump leachate solutions.
Thus, for example, U.S. Pat. No. 2,296,423 discloses a method whereby acid solutions containing iron (or iron and aluminum) are subjected to high temperature and pressure in an autoclave to hydrolize sulfates of ferric iron an aluminum and precipitate basic salts with a simultaneous generation of free acid. According to the patent, oxidation of iron to the ferric state is promoted by the direct injection of oxygen into the solution while the solution is at high temperature and pressure. The patent teaches the addition of an alkali salt, e.g., sodium sulfate or sodium chloride, to promote the precipitation. Soluble iron oxides are added to partly consume the free acid generated in the autoclave operation. The precipitate formed under the conditions taught by the patent is disclosed to be a double basic salt of alkali metal and iron, Na.sub.2 SO.sub.4 .3 Fe.sub.2 (OH).sub.4 .SO.sub.4, somewhat analogous to, but apparently different from the natural mineral Natrojarosite, NaFe.sub.3 (SO.sub.4).sub.2 (OH).sub.6. The patent further teaches that if aluminum is present in the solution both the iron and aluminum can be nearly completely precipitated as a complex basic iron aluminum alkali sulfate.
U.S. Pat. No. 3,434,947 is directed to separation of iron from zinc sulfate solutions produced in hydrometallurgical leaching of "Calcine", a roasted sulfide ore concentrate. The iron is precipitated in the presence of K.sup.+, Na.sup.+, NH.sub.4 .sup.+ ions in a concentration of 1/10 to 1/4 the amount of the iron content in g/l. Ferrous ion is oxidized to ferric ion by MnO.sub.2, and the solution is partly neutralized with ZnO prior to the precipitation. The basic iron sulfate precipitated is described in related U.S. Pat. No. 3,684,490 as being jarosite, but the source of the K.sup.+, Na.sup.+ and/or NH.sub.4 .sup.+ is an unnamed salt.
Sideronatrite, Metasideronatrite, Natrojarosite, and Ammoniojarosite are found in nature (See Palache, C.; Berman, H.; Frondel, C.; Dana's System of Minerology, Vol. II, John Wiley & Sons, 7th Ed., 1951, pp. 562, 563, 603, 604). However, the conditions under which formation of these compounds occurred in nature is unknown. Scharizer, in Zs.Kr., Vol. 41 (1906), p. 215, reports formation of Sideronatrite by slow precipitation at room temperature over a period of months. Mellor, "A Comprehensive Treatise of Inorganic & Theoretical Chemistry", Vol. 14, p. 345 (1935), Longmanns Green & Co., reports on work by Skrabal, A., Zeit. anorg. Chem., Vol. 38 (1904), p. 319, as preparing Sideronatrite under conditions of high Na, Fe and SO.sub.4 concentration by heating sodium and ferric sulfate in the presence of suluric acid on a hot plate at an unknown temperature.
Therefore, there is a great need for a process for control of SO.sub.x emissions from pyrometallurgical smelter operations, both retrofit and new plants. There is also a need for a process which can employ sodium, yet in which the sodium sulfite/sulfate waste material can be a useful product elsewhere in the overall copper operations. There is also a need for improving the efficiency of heap and dump leaching, and preventing the plugging of the dumps by formation of ferrous and ferric salt compounds within the dump which reduces efficiency.